The present invention relates to a melt blow die and, more particularly, to a melt blow die of the type having a plurality of capillary tubes through which melt of a thermoplastic resin is extruded and the extruded melt is drawn and spun into fine fibers by means of a gas which is blown through orifices formed around the capillary tubes onto the melt immediately after the extrusion.
Melt blow dies have been known as disclosed, for example, in Japanese Patent Publication No. 44470/1983 (based on U.S. Ser. No. 242,506) and Japanese Patent Laid-Open publication No. 159336/1981 (based on U.S. Ser. No. 138,860) More specifically, the melt blow die disclosed in Japanese Patent Publication No. 44470/1983 is constituted by a die tip having a triangular cross-section, at least one array of capillary tubes soldered to the die tip, and gas plates disposed above and below the die tip leaving suitable clearances constituting gas orifices. Disclosed also is a melt blow die in which one ends of the capillary tubes arranged in an array are rigidly held by a die block so that the capillary tubes are cantilevered, and a pair of gas plates are disposed above and below the the capillary tubes so as to define orifices. In operation, a gas is blown onto fibers in molten state immediately after the extrusion at a predetermined angle with respect to the extruded fibers, through the gas orifices which are formed between the gas plates and the die tip or between the gas plates and the free ends of the capillary tubes.
On the other hand, Japanese Patent Laid-Open Publication No. 159336/1981 discloses a melt blow die in which a plurality of capillary tubes arranged in the form of a matrix are inserted into tiny holes of a screen mesh such that the ends of the capillary tubes project through the screen mesh, and the spaces around the capillary tube are utilized as gas orifices through which a gas is blown so as to draw the melt of the thermoplastic resin extruded from the capillary tubes thereby forming fine fibers.
In general, a melt blow die has to be designed such that the size of the orifices is uniform along the array or arrays of the capillary tubes so that a uniform distribution of the gas flow rate is obtained along the array or arrays of the orifice. This is because any irregularity in the drawing gas flow rate results in non-uniform quality of the product fibers.
Therefore, the first-mentioned type of melt blow die disclosed in Japanese Patent Publication No. 44470/1093 essentially requires that the gas plates and the die tip are extremely precisely finished and assembled in order to attain the desired uniformity of the gas orifice size. In addition, the size of the gas orifice tends to become non-uniform as a result of thermal strain or secular change during long use, even if the assembly of the die has been initially completed such as to provide a uniform size of the gas orifice. The second-mentioned type of the melt blow die having cantilevered capillary tubes suffer from disadvantages in that the free ends of the cantilevered capillary tubes tends to be irregularly positioned and to vibrate when blown by the drawing gas.
In order to attain uniform size of gas orifices in the melt blow die proposed in Japanese Patent Laid-Open Publication No. 159336/1981, it is necessary that the screen mesh be finished in such a way as to have tiny holes arranged at an extremely precise pitch. It is not easy to form such a screen mesh. In addition, it is necessary that the capillary tubes are inserted one by one into the designated tiny holes of the screen mesh, in order to correctly locate the capillary tubes so as to attain uniform size of the gas orifices. Such a work is extremely laborious.
For these reasons, it has been difficult to disassemble and reassemble a melt blow die for the purpose of inspection.
Accordingly, a first object of the present invention is to provide a melt blow die which can easily be disassembled and re-assembled and which have high degree of uniformity of the size of gas orifices around the capillary tubes.
The melt blow die of the type proposed in Japanese patent Publication No. 44470/1983 encounters another problem in that it requires laborious work for detaching and attaching the capillary tubes when cleaning or renewal of the capillary tube is required, because the capillary tubes are directly fixed to the die block by soldering. This problem is serious particularly in the case where a plurality of arrays of the capillary tubes are arranged densely. Namely, in such a case, a plurality of arrays of capillary tubes are soldered successively so that the heat applied during soldering of an array is transferred to the preceding array which has been soldered, due to small pitch of arrays and due to high heat conductivity of the die block which is usually made from a metal. In consequence, the solder of the preceding array is molten again to allow the capillary tubes of this array to be set irregularly.
It would be possible to solder a plurality of arrays of capillary tubes at once. However, it is quite difficult to solder a plurality of arrays of capillary tubes in one time without impairing regularity of arrangement of the capillary tubes.
Accordingly, a second object of the present invention is to provide a melt blow die which is improved in such a way as to facilitate replacement of the capillary tubes and to enable a plurality of arrays of capillary tubes at a small pitch.